Crystalline polypropylene film has excellent mechanical properties such as tensile strength, rigidity, surface hardness, impact resisting strength and cold resistance, optical properties such as gloss and transparency and food sanitation properties such as nontoxicity and odorlessness. Crystalline polypropylene film is widely employed in the field of, especially, food.
However, crystalline polypropylene film has a high heat sealing temperature with a small range, so that there occur problems of poor welding of heat seal portions and fusion thereof. Thus, for solving the above heat sealing problems of the crystalline polypropylene film, it is common practice to laminate mold a resin layer which is to become a heat seal portion to a surface of the crystalline polypropylene film.
A variety of resins have been studied for use in the formation of the above resin layer. It is required that the resin for use in the formation of the resin layer be capable of:
(1) permitting heat sealing at temperatures appreciably lower than that of the substrate; PA1 (2) having high heat seal strength; PA1 (3) having desirable adherence to the substrate; PA1 (4) having transparency that is identical with or greater than that of the substrate; PA1 (5) being free from blocking during the storage; PA1 (6) being free from sticking to bag forming and filling packing jigs; PA1 (7) having high scratch resistance; and PA1 (8) exhibiting minimal change of the heat seal strength irrespective of the passage of time. PA1 the above propylene/1-butene random copolymer (A): PA1 (1) comprising 50 to 95 mol % of structural units derived from propylene and 5 to 50 mol % of structural units derived from 1-butene; PA1 (2) exhibiting a melt flow rate (measured at 230.degree. C. under a load of 2.16 kg in accordance with ASTM D 1238) of 0.1 to 40 g/10 min; PA1 (3) having a molecular weight distribution (Mw/Mn), measured by gel permeation chromatography (GPC), of up to 3; and PA1 (4) having a B-value, being a parameter indicating a randomness of copolymer monomer chain distribution, of 1.0 to 1.5, and PA1 the above low-density polyethylene (B): PA1 (1) exhibiting a melt flow rate (measured at 190.degree. C. under a load of 2.16 kg in accordance with ASTM D 1238) of 1 to 30 g/10 min; and PA1 (2) having a density of not greater than 0.940 g/cm.sup.3. PA1 the propylene/1-butene random copolymer (A): PA1 (4) has a B-value, being a parameter indicating a randomness of copolymer monomer chain distribution, of 1.0 to 1.3; PA1 (5) has a melting point (Tm), measured by a differential scanning calorimeter, of 60 to 140.degree. C., PA1 the above melting point, Tm, and a content of 1-butene structural units, M (mol %), satisfying the relationship: EQU -2.6M+130.ltoreq.Tm.ltoreq.-2.3M+155; and PA1 (6) has a crystallinity measured by X-ray diffractometry, C(%), this crystallinity and the content of 1-butene structural units, M (mol %), satisfying the relationship: EQU C.gtoreq.-1.5M+75, and PA1 the above low-density polyethylene (B): PA1 (3) exhibits a melt flow rate (measured at 190.degree. C. under a load of 2.16 kg in accordance with ASTM D 1238) of 1 to 25 g/10 min; and PA1 (4) has a density of 0.915 to 0.935 g/cm.sup.3. PA1 the above olefin polymerization catalyst comprising: PA1 (a) a transition metal compound represented by the general formula: ##STR1## PA1 M represents a transition metal of Group IVa, Va or VIa of the periodic table; PA1 each of R.sup.1 and R.sup.2 independently represents a hydrogen atom, a halogen atom, a hydrocarbon group having 1 to 20 carbon atoms, a halogenated hydrocarbon group having 1 to 20 carbon atoms, a silicon-containing group, an oxygen-containing group, a sulfur-containing group, a nitrogen-containing group or a phosphorus-containing group; PA1 R.sup.3 independently represents a secondary or tertiary alkyl group having 3 to 20 carbon atoms or an aromatic group having 6 to 20 carbon atoms; PA1 R.sup.4 independently represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms; PA1 each of X.sup.1 and X.sup.2 independently represents a hydrogen atom, a halogen atom, a hydrocarbon group having 1 to 20 carbon atoms, a halogenated hydrocarbon group having 1 to 20 carbon atoms, an oxygen-containing group or a sulfur-containing group; PA1 Y represents a divalent hydrocarbon group having 1 to 20 carbon atoms, a divalent halogenated hydrocarbon group having 1 to 20 carbon atoms, a divalent silicon-containing group, a divalent germanium-containing group, a divalent tin-containing group, --O--, --CO--, --S--, --SO--, --SO.sub.2 --, --NR.sup.5 --, --P(R.sup.5)--, --P(O) (R.sup.5)--, --BR.sup.5 -- or --AlR.sup.5 -- (provided that R.sup.5 represents a hydrogen atom, a halogen atom, a hydrocarbon group having 1 to 20 carbon atoms or a halogenated hydrocarbon group having 1 to 20 carbon atoms), PA1 (b) an organoaluminum oxy compound (b-1) and/or a compound (b-2) capable of reacting with the transition metal compound (a) to thereby form an ion pair, and optionally PA1 (c) an organoaluminum compound. PA1 50 to 95 mol %, preferably, 55 to 93 mol % and, still preferably, 60 to 90 mol % of structural units derived from propylene, and PA1 5 to 50 mol %, preferably, 7 to 45 mol % and, still preferably, 10 to 40 mol % of structural units derived from 1-butene. PA1 wherein P.sub.1 and P.sub.2 represent first monomer and second monomer content fractions, respectively, and P.sub.12 represents the proportion of (first monomer)-(second monomer) chains to all bimolecular chains. PA1 (a) a specified transition metal compound (metallocene compound), PA1 (b) an organoaluminum oxy compound (b-1) and/or a compound (b-2) capable of reacting with the transition metal compound (a) to thereby form an ion pair, and optionally PA1 (c) an organoaluminum compound. PA1 polyolefins such as polypropylene and poly-1-butene; PA1 polyamides such as nylon 6 and nylon 66; and PA1 polyesters such as polyethylene terephthalate and polybutylene terephthalate. These may be in the form of a nonoriented film or may be in the form of a uniaxially or biaxially oriented film. A biaxially oriented polypropylene film is especially preferred.
A propylene/1-butene random copolymer is used as the resin capable of forming the resin layer which is to become a heat seal portion. It is known that this copolymer has excellent transparency and low-temperature sealing properties and has relatively good blocking resistance.
In the lamination of the propylene/1-butene random copolymer onto a surface of the crystalline polypropylene film, increasing the laminating speed is likely to cause molding problems such as increases of surging (film fluctuation) and neck-in. For solving these laminating problems, it was proposed to blend low-density polyethylene with the propylene/1-butene random copolymer (see Japanese Patent Laid-open Publication No. 54(1979)-120656).
However, in recent years, the progress in increasing the speed of wrapping machines is remarkable, so that there is a strong demand in the art for the development of a propylene/1-butene random copolymer composition suitable for a laminate molding of a composite film which not only permits sealing at lower temperatures, namely, has excellent low-temperature sealing properties, but also is excellent in blocking resistance and further for the development of the above composite film.